Miniature poodle, Irish setter and Norwegian elkhound dogs will be studied to determine common disease mechanisms shared by these three different photoreceptor degenerations. Electroretinography and ultrastructure will assess function and integrity of rods and cones to characterize and stage the diseases. Autoradiography will determine if outer segment renewal defects occur and the correlation to the photoreceptor degenerative process. Cyclic nucleotide determinations will correlate abnormalities in cyclic nucleotide metabolism and photoreceptor degeneration. Studies of different retinal areas will determine regional variations of the disease in terms of morphology, outer segment renewal and cyclic nucleotide metabolism.